Ordinary dynamoelectric machines of the permanent magnet type have a rotor assembly comprising a plurality of permanent rotor magnets arranged radially around a drive shaft that rotate with the drive shaft about a stator assembly that comprises a plurality of stator poles and a stator winding. The rotor magnets have a fixed axial alignment that generally coincides with the axial position of the stator poles. The rotor magnets have a fixed radial alignment that is inside the radial position of the stator poles for machines of the conventional type or outside the radial position of the stator poles for machines of the “inside out” type.
Since the rotor magnets have a fixed radial and axial alignment with respect to the stator poles, when used as a generator the electrical potential that the dynamoelectric machine generates is primarily proportional to the rotational speed of the rotor assembly and the power consumed by an electrical load. As a generator, a primary problem with this class of machines is that there is no convenient way to regulate the generated electrical potential that may vary with rotational speed and load variations, unlike dynamoelectric machines with a rotor winding that may control rotor winding current from an exciter for regulation.
Similarly, when used as a motor driven by source of electric power, the back electromotive force (EMF) that the dynamoelectric machine generates subtracts from the electrical potential of the power source. The power source must supply increasing electrical potential for increasing speed at constant torque. Eventually, the power source cannot supply additional potential and then the output torque of the dynamoelectric machine falls with increasing speed until no further torque is achievable.
As a motor, a primary problem with this class of machines is that there is no convenient way to regulate the generated back EMF that increases with rotational speed, unlike dynamoelectric machines with a rotor winding that may control rotor winding current from an exciter to reduce back EMF at high speeds and thereby achieve high speed output torque.
Consequently, ordinary dynamoelectric machines of the permanent magnet type used as a generator may produce a lower electrical potential than required when operated at a slower rotational speed than a desired operational speed and produce too much potential when operated at a faster rotational speed than the desired operational speed. Such variations in potential can cause hazardous conditions or damage to electrical components that comprise the electrical load. Ordinary dynamoelectric machines of the permanent magnet type used as a motor have poor torque characteristics at high speeds when they have a design that requires high torque at low speeds.
U.S. Ser. No. 11/405,743, filed 17 Apr. 2006 by Himmelmann et al., owned by the assignee of this application and incorporated in this application by reference, describes a new type of permanent magnet dynamoelectric machine that has at least one permanent magnet rotor that is axially displaceable by hydraulic means to achieve a variable rotor field characteristic. An active or passive control system may control the rotor displacement of this variable field permanent magnet (VFPM) machine to regulate the output potential in generator service or torque in motor service. U.S. Ser. No. 12/034,759, filed 21 Feb. 2008 by Himmelmann et al., owned by the assignee of this application and incorporated in this application by reference, describes an example of an active control system for such a VFPM machine.
Some applications may require a pair of such VFPM machines to drive a single drive shaft, either in a direct-coupled torque summing mode or a speed-summing mode by way of a gearbox, such as an epicyclic gear train. In either case, the minimum length of the entire assembly would be twice the length of each machine at maximum rotor displacement plus the length of a coupling gearbox, if any. Unfortunately, such an assembly may be too long for certain applications, such as part of a power drive train for a tracked vehicle wherein each track may have one machine as a propulsion motor and the other machine as a steering motor.